Ethylene polymerization process



June 4, 1963 J. F. ERDMANN ETAL 3,092,614

ETHYLENE POLYMERIZATION PROCESS Filed Feb. l5. 1962 EES s lNEI) 83dNOISHBANO) INVENORS JOHN F. ERDMANN CHARLES R. WELTER .9i/.Ha @KALQ s`ATTORNEY 3,092,614 ETHYLENE POLYMERIZATION PROCESS John F. Erdmann,Charleston, and Charles R. Weiter, South Charleston, W. Va., assignorsto Union Carbide Corporation, a corporation of New York Filed Feb. 15,1962, Ser. No. 176,845. 6 Claims. (Cl. 2611-949) This invention relatesto an improved process for polymerizing ethylene either alone or in admixture with one or more polymerizable compounds. This application 1s -acontinuation-impart application of Serial No. 807 377 ined April 20,1959 now abandoned.

It is known to polymerize ethylene using oxygen or air as a catalyst, asdisclosed initially in United States Patent 2,153,553 and United StatesP-atent 2,188,465 wherein normally solid polymers of ethylene areproduced by subjecting ethylene to temperatures of from about 100 C. to-about 400 C. at pressures above 500 atmospheres in the presence ofoxygen. It isi-also known to polymerize ethylene in the presence ofperoxidic catalysts :at elevated temperatures and pressures, forexample, as disclosed in United States Patent 2,409,996. In this latterpatent ethylene is polymerized in a stirred yautoclave .at 100 C. to 275C. at pressures of from about 400 to 1,000 latmospheres or above in thepresence of alkyl hydroperoxide catalysts; the polymerization ispreferably carried out using monomeric ethylene of high purity having anoxygen content of less than about 10 parts per million.

It has now been found that a. mixture of oxygen and tertiary butylhydroperoxide offers decided advantages over either oxygen alone ortertiary butyl hydroperoxide alone as a catalyst for the polymerizationof ethylene. Furthermore, it has now been found that the characteristicsof this system of mixed catalysts are decidedly different than expectedfrom merely averaging the behavior of the individual catalysts; that adecidedly synergistic eifect is observed, which results in unexpectedand unforseen increases in conversion Ito polymer, greater ease incontrolling the polymerization reaction, lower polymerizationtemperatures without decreasing the conversion, and improved quality ofthe polyethylene produced including production of higher molecularweight polymer than could be produced when using either catalyst alone,as evidenced -by lower melt indices of the resins than can be obtainedby the use of either catalyst by itself. The melt index, as is known,i-s a measure of the molecular weight of rthe polymer, and is inverselyproportional to the molecular weight.

Molecular oxygen as a 'catalyst in ethylene polymerizations has certaininherent disadvantages, foremost among which is that it is relativelyinactive at low temperatures. Thus it is diicult, if not impossible, toproduce linear, high molecular weight polyethylene since at the elevatedtemperatures ordinarily required with oxygen as catalyst, chainbranching and lower molecular weight resins are favored. In addition,the concentration of oxygen used as catalyst must be carefullycontrolled, since increasing the oxygen catalyst content may suddenlyaccelerate the reaction to such an extent that the heat of reaction isnot dissipated fast enough, and the reaction temperature may even becomesuciently high so that the resin turns brown or decomposes tocarbonaceous material; whereas, if the oxygen concentration is too low,the productivity drops with la resulting loss of profits due to lowconversion.

Tertiary butyl hydroperoxide :alone is lan eifective catalyst inproducing high conversion of ethylene to polyethylene, but it initiatesa polymerization reaction whichis rapid and diflicult to control. Whenthis catalyst is used by itself, frequent decompositionsoccur, and poly-States arent FPP??? ethylene having low and widely varying molecularweights is often obtained. In addition, tertiary butyl hydroperoxiderequires an even higher initiation temper-affine than oxygen thus makingit even still more diliicult to obtain high molecular weightpolyethylene.

The .process of this invention is preferably carried out in lacontinuously operated tubular vreactor at temper-atures of from about C.-to about 250 C., preferably from about C. to about 200 C., and atpressures above about 15,000 p.s.i., preferably from about 25,000 p.s.i.to 4about 40,000 p.s.i., and may be as high as 100,000 p.s.i. or more.Use of 'the mixed oxygen tertiary butyl hydroperoxide catalysts of thisinvention under such narrow conditions results in increased conversionsabove the conversions obtained when using either catalyst alone. Inaddition, the molecular weights are increased as evidenced by the factthat the melt index of the polyethylene produced with the mixedcatalysts can be as Ilow as 0.001 decigr-am per minute even at 20percent conversions, whereas when either catalyst is used alone, themelt index seldom, if ever, drops below r0.01 decigr-am per minute atconversion levels above about 17 percent.

The use of the mixed catalyst systems of this invention overcome, to 4alarge extent, some of the diiculties encountered when oxygen or t-butylhydroperoxide are used singly. Further, the mixed catalyst systemsresult in higher conversion to polymer having a specific melt in- !dex.'Ihis is clearly seen from the following table based on the drawing.

Table I Air or t-Butyl Mixed Melt Index of Polyethylene HydroperoxideCatalyst Alone The increased molecular weights and higher conversionsachieved with our catalyst mixtures are plotted 1n the drawing.Examination of the results plotted in the drawing shows that the maximumconversion of ethylene to polymer using either oxygen or tertiary butylhydroperoxide alone was about 23 percent and that the melt indicesvaried only from about 0.007 to about 3,0 declgrams per minute at theseconversions. These results are encompassed within the area in which thelines slant from top left to bottom right. However, when the mixedcatalyst compositions of this invention are used, the conversion topolymer can exceed 30 percent, and Ithe melt indices at such conversionscan vary from lbelow 0.001 up to about 100 decigrams per minute,depending on the catalyst composition and concentration. The linessloping from top right `to bottom left encompass the scope obtainablewith the catalyst mixtures of this invention. From the drawing it isclearly evident that oxygen or tertiary butyl hydroperoxide alone couldnot produce the high molecular weight, low melt index polymersobtainable by the catalyst mixtures of this invention when operatedwithin the critical reaction conditions herein set forth. The fact thatthe catalyst mixtures of this invention do produce such low melt indexresins 'at lower temperatures and at such good conversions wasunexpected, and could not have been predicted or anticipated from theprior art, since generally a decrease in temperature causes a decreasein conversion. It is seen that the catalyst compositions of thisinvention extend the range of melt indices available and inf crease theconversions obtainable over those values which result when eithercatalyst is used alone; such results could not be foreseen or expected.

The total concentration of catalyst mixure used 1s 'dependent on thedimensions of the tubular reactor and will depend upon the ratio of thevolume ofthe reactor to the cooling surface, since these factors willaffect the feed rate. 'Ilhe total concentrations will also depend uponthe temperature at which .the polymerization 1s Vcarried out, the rateof ethylene throughout (which ca n be varied from 500 to 4,000 poundsper hour per cubic foot for higher), and the heat rtransfer eifected`through the heavy reactor walls required for carrying out reactions atsuch elevated pressures. Thus the total catalystV concentrationV can beused to a large extent to control the reaction and preventdecomposition. This vtotal catalyst concentration can be varied fromabout 51 to about 600 parts per million on a molar basis based Von theethylene charge, and is preferably from about 51 to about 150 parts permillion. The catalyst concentration is dened in this `application as themoles of catalyst, or 1ndividual component, per million moles ofethylene.

The concentration of the oxygen component present in the catalystmixture can be varied from about 50 to about 500 parts v.per millionbased on the ethylene charged, and is preferably from about 50 to about135 parts per million. The tertiary butyl hydroperoxide content can bevaried from about 1 to about 100 partsgper million, and is preferablyfrom about 2 to about 15 parts per million. The ratio of oxygen totertiary butyl hydroperoxide in the catalyst mixture is not critical,and can be varied overa wide range; ratios of from about 40:1' to about2:1 have been found suitable, and should preferably be above about 7: 1.

'Y The tertiary butyl hydroperoxide is added to fthe polymerizationreaction as a solution in an inert solvent such as benzene, toluene,iso-octane, and the like. The inert solvent can be present in thereactor -to the extent of from about 0.1 to about mole percent based onthe ethylene being charged. Y 1 Y The purity of the ethylene Vis notcritical and can vary from about 90 to 99.9 percent pure ethylene: Amongthe most common impurities found in commercially. available ethylenelare Ithe low molecular weight'hydroc'arbons such -as methane, ethane,propane, :traces of propylene and traces of acetylene, with presence ofthe unsaturated hydrocarbons being less desirable Ithan presence of thesaturated hydrocarbons.

'Il-he mixed oxygen/ tertiary butyl hydroperoxide catalyst compositionsof this invention can also be used to polymerize ethylene'mixturescontaining chain transfer agents or other polymerizable olefinicallyunsaturated compounds; for example, ketones, alcohols, alk-anes,cycloalkanes, arylalkanea :alkenes, vinyl esters, vinyl ethers, acrylicacid and its esters, et cetera.

The polymerization .lof ethylene by .the process of this invention Icanbe conveniently carried out in a continuously-operated tubular reactorabout 50 to 150 feet long, or'longer, having an .inside diameter of fromabout BAG to 1/2 inch, ormore. The ethylene was deoxygenated to lessthan 10 parts per million oxygen, and then filtered dry -air wasmeteredinto the deoxygenated ethylene to provide the desired oxygencontent. This ethylene feed, containing the oxygen catalyst, wascompressed to 3,000 to 5,000 p.s.i. and cooled to liquefy. A solution ofItertiarybutyl hydroperoxide in an inert solvent was then metered intothe liquid ethylene feed, which was further compressed by high pressurepumps to about 30,000 p.s.i. before entering the jacketed high pressure.tubular reactor. The ethylene feed wasY passed through the reactor at aspace velocity of from about 500 to 4,000 pounds per hour per cubicfoot. The disch-arge mixture from/the reactor was quenched in water, theunreacted ethylene was recovered for recycling, and the polymer producedwas filtered from the aqueous slurry and dried.

YIn this specification the physical properties were determined accordingto the following procedures:

The polyethylenes produced by the process of this invention arecomparable in physical and chemical properties, such as stress crackresistance, brittleness, and infrai red analysis forV carbonyl :andunsaturati-on, to the polyethylenes presently available on a .commercialscale. The resins so produced find application in the same fields ofcommerce, for example, inmoldings, extrusions, films, coatings, fibers,et cetera, but because of theirimproved properties they are moredesirable.

The following examples further serve to illustrate this invention. Inthese examples the ethylene was deoxygenated to less than 10 parts permillion oxygen, and then filtered air was meteredinto the desired oxygenconcentration. After partial compression, as described above, a benzenesolution of tertiary butylY hydroperoxide Was metered in to provide -thedesired concentration of tertiary butyl hydroperoxide. Thecatalyst-containing ethylene feed was then reacted in a continuoustubular reactor about 60 feet long, having an inside diameter of aboutf; inch.

Table II summarizes :the reaction conditions, yields and the propertiesof the resins produced using a feed rate of from 2,000 .to 3,000 poundsper hour per cubic foot, ternperatures of from C. -to 225 C., reactorpressure of about '30,000 p.s.i., constant oxygen concentrationsV ofabout 60 parts per million and varying tertiary butyl hydroperoxidecontents, as indicated. The improved eiect of .the mixed catalyst isstrikingly brought out when the yields obtained with the mixed catalystof this invention are compared with Vthe yields obtained with oxygenalone at the same temperature, and with a control at 175 C. using 30parts per million tertiary butyl hydroperoxide alone.

yFrom Table II it can be seen that resins having the un-V expected lowermelt indices, or highermolecular weights, are consistently obtained atcommercially practical conversions at polymerization temperatures offrom about 170 C. to about 200 C. with the catalyst mixtures hereindisclosed.

Further,it is seen that at 170 C. the use of 61 parts per million oxygenas the sole catalyst gives but 3.3 percent conversion to polymer;however, the addition of only Z parts per million tertiary butylhydroperoxide to 61 parts per million oxygen surprisinglyandunexpectedly increases conversion to 17.8 percent, and even moresurprisingly, the product produced has a higher molecular weight. Thisunexpected increase in conversion and in molecular weight is unobvious,and could not have been predicted as it was found that at C. the use of5.5 parts per million of tertiary butyl hydroperoxide gives but a 7.8percent conversion to polymer.

The unexpected and unobvious results are apparent at the highertemperatures, preferably at temperatures up to about 200 C. Thus at 175C. the use of 62 parts per million of oxygen alone as the catalystresults in a conversion 4'of only 15.9 percent. If, instead ofoxygen,one uses 30 parts per million tertiary butyl hydroperoxide alone,which is roughly equivalent to about 120 parts per million of oxygen,one gets a conversion of only 4 percent to resin having the same meltindex as obtained at 175 C. with about 60 parts per million oxygen.However, if one adds only 2.2 pants per million of tertiary butylhydroperoxide to the original 60 parts per million of oxygen, theconversion increases to 20.0 percent, and themelt index unexpectedlydecreases to less :than 0.001. The use of increased amounts of tertiaryvbutyl hydroperoxide further increases the conversion Withoutexcessively increasing the melt index disproportionately when all theother variables are maintained constant. Such unexpected increases inconversion are unobvious when it is recalled `that at temperatures belowabout 200 C. neither catalyst by itself will give a conversion muchabove about l5 percent.

The unobvious and unexpected improvements in conversion and melt indexoccur over the critical temperature range set forth in thisspecification. At 180 C. it is seen that conversion is doubled yto 28.9percent by the addition of only 8.2 parts per million tertiary butylhydroperoxide to 62 parts per million of oxygen catalyst. Similarresults are obtained at 185 C., 190 C. and 205 C., whereas at 6 lystmixture comprising from yabout 50 to about 500 parts per million of:oxygen :and from about 1 to about 100 parts per million of tertiarybutyl hydroperoxide, said catalyst concentration being `on a molar basisbased on the ethylene charged.

4. In a bulk tubular reactor process for polymerizing ethylene iat a:temperature between about 170 C. and 200 C. and a pressure above 15,000p.s.i., the improvement which comprises conducting the polymerization inthe presence of from about 51 to about 15 0 parts per million `of acatalyst mixture comprising from about 50 to 1-35 parts per million ofoxygen 4and -from about 2 to about parts per million of tertiary butylhydroperoxide, said catalyst concentration being on a molar basis based225 C. the effect 1s not as pronounced. 15 on the ethylene charged.

Table II Example 1 2 3 4 5 6 7 8 9 10 Catalyst, ppm.:

Oxygen 63 64 61 62 62 G1 t-Butyl hydroperoxide.- 2. 1 8. 7 0 2, 0 8. 2 0Reactor temperature, C. 205 205 190 190 190 185 Conversion, percent 24.O 24. 3 14. 5 15. 8 26. 5 14. 5 Increase in conversion, percent:

Over oxygen al 57 9 83 Nlelt index, dgnL/Inn 3. 3 28 0.38 0. 04 6. 80.12 Density g./cc 9180 H. 9138 9263 .9228 .9336 9260 Tensileproperties:

Stifness, p.s.i. X 10-3 18 ll 33 22 12 31 Yield point, p.s.i. 1, 4001,160 2, 210 1, 680 1, 110 1, 840

Ultimate tensile, p.s.i 1, 720 980 2, 390 1, 335 1, 110 2, 460

E longation, percent 680 157 775 155 113 885 Example 11 12 13 14 15 1617 18 19 20 Catalyst, ppm.:

xygen 60 61 62 62 62 62 61 t-Butyl hydroperoxide 2. 1 8. 2 0 8. 2 0 2. 22. 0 Reactor temperature, C. 185 185 180 180 175 175 170 Conversion,percent 16. 4 28. 5 14. 3 28. 9 15. 9 20.0 17. 8 Increase in conversion,percent:

97 102 26 440 3. 0 0. 06 0. 73 0. 02 001 001 9260 9250 9178 9258 92429246 Tensile properties:

Stitness, p.s.i. X 103 12 15 16 35 23 27 Yield point, psi- 1, 210 1,9501, 400 1, 990 1,850 1, 910

Ultimate tensile, p 1,035 2, 400 1, 730 2, 900 2, 550 3, 425

Elongation, percent. 425 170 640 740 785 480 585 What is claimed is:

1. In a bulk tubular reactor continuous process for polymerizingethylene at a temperature between about 160 C. and 250 C. and a pressureabove 15,000 psi., the improvement which comprises conducting thepolymerization reaction in the presence of a catalyst mixture comprisingfrom about to about 500 parts per million of oxygen and from about l toabout 100 parts per million of tertiary butyl hydroperoxide, saidcatalyst concentration being on a molar basis based on the ethylenecharged.

2. In a bulk tubular reactor process for polymerizing ethylene at .atemperature between about 160 C. and 250 C. :and a pressure above 15,000psi., the improvement which lcomprises conducting the polymerizationreaction in the presence of from about 51 to about 600 parts per millionof a catalyst mixture comprising from about 50 to 500 parts per millionof oxygen and yfrom about 1 to about .100 parts per million of tertiarybutyl hydroperoxide, said catalyst concentration being on a molar basisbased lon the ethylene charged.

3. In la bulk tubular reactor process for polymerizing ethylene at Iatemperature between about 170 C. and 200 C. .and a pressure betweenabout 25,000 p.s.i. and

40,000 ps i., the improvement which comprises conducting thepolymerization reaction in the presence of a cata- 5. In (a bulk tubularreactor process for polymerizing ethylene at a tempera-ture betweenabout s170 C. and 200 C. :and a pressure between Iabout 25,000 psi. and40,000 p.s.i., the improvement which comprises conducting thepolymerization in the presence of from about 51 to about parts permillion of a catalyst mixture comprising `from about 50 to about 135parts per million of oxygen land from about t2 to labout 1 5 parts permillion of tertiary butyl hydroperoxide, said catalyst concentrationbeing `on la molar basis based on the ethylene charged.

6. In a bulk tubular reactor process for polymerizing ethylene at atemperature between about C. 1and 200 C. and 'a pressure between about25,000 p.s.i. and 40,000 p.s.i., the improvement which comprisesconducting the polymerization in the presence of a catalyst mixturecomprising ttrom about 50 :to .about 135 parts per million of oxygen andtrom about 2 to about 115 parts per million of tertiary butylhyd-ropenoxide, said catalyst concentration being on a molar basis basedon the ethylene charged.

References Cited in the le of this patent FOREIGN PATENTS 591,335iGrea't Britain Aug. 14, 1947

1. IN A BULK TUBULAR REACTOR CONTINUOUS PROCESS FOR POLYMERIZINGETHYLENE AT A TEMPERATURE BETWEEN ABOUT 160*C. AND A PRESSURE ABOVE15,000 P.S.I., THE IMPROVEMENT WHICH COMPRISES CONDUCTING THEPOLYMERIZATION REACTION IN THE PRESENCE OF A CATALYST MIXTURE COMPRISINGFROM ABOUT 50 TO ABOUT 500 PARTS PER MILLION OF OXYGEN AND FROM ABOUT 1TO ABOUT 100 PARTS PER MILLION PF TERITARY BUTYL HYDROPEROXIDE, SAIDCATALYST CONCERTRATION BEING ON A MOLAR BASIS BASED ON THE ETHYLENECHARGED.